First - they're all beautiful! Love the backgrounds (fits with how I imagined the planet), the model is gorgeous (and love the protective goggles, gotta protect herself from the sunlight somehow, right?).

Now, nitpicking:

On the first two covers it seems like the model and the buildings and the UFO (love the round UFO! lol) in the background seem slightly more stark and colorful than the landscape behind them. Don't know how, but can you make them blend more together? Only slightly. Especially the building and the UFO, they seem to stick out of the background a little. I love that the font is colorful and clear, though.

Dead and Buryd - did you try having the model more to one side, rather than in the middle? Now she's got two items behind her, the prison and the pillar, and I'm just wondering if it's a bit crowded. How about having either the prison or the pillar on the backcover, and one on the front?

Fight or Flight - same thing here, the model has three things behind her, the UFO, a moon and a mountain (I think). Seems a bit crowded, any chance of taking something out or moving them more to the side? For some reason I do like that her head brushes against the title on all four.

Rack and Ruin - I think this is my favorite one. Absolutely love the colors, although they don't match the other three. Is this just a test or did you mean to have them this way? I think they should all match in color. Love her clothes, love the background. Love the ruins! The font color should match the others too.

Crash and Burn - love it, don't have much to nitpick, lol. Apart from the font color, should match the others. Author name is white on snowy background, but I guess it should be readable.